Description of EUV Lithography
Associated with miniaturization of semiconductor devices in recent years, EUV lithography using, as a light source, EUV having a wavelength around 13.5 nm has been proposed. EUV lithography has to be performed in vacuum since the wavelength of a light source is short and the light absorption property is very high with EUV lithography. In addition, the refractive indices of most substances are values slightly smaller than 1 in the wavelength range of EUV. Therefore, the conventionally used transmission type dioptric system cannot be used in EUV lithography, and a catoptric system is used. Thus, it is necessary to us a reflective mask as a photomask (hereinafter, also referred to as mask) that becomes the master copy, since the conventional transmissive mask cannot be used.
Description of EUV Mask and Blank Structure
A reflective mask blank, which becomes the basis of such reflective mask, is obtained by sequentially forming, on a low thermal-expansion substrate, a multilayered reflective layer having high reflectance against the wavelength of an exposure light source, and an absorption layer configured to absorb the wavelength of the exposure light source, and further forming, on the reverse surface of the substrate, a reverse-surface conductive film for electrostatic chucking in an exposure machine. There are also EUV masks with a structure having a buffer layer between the multilayered reflective layer and the absorption layer. When processing the reflective mask blank into a reflective mask, the absorption layer is partially stripped together with a buffer layer if the structure has a buffer layer, using EB lithography and etching technology to form a circuit pattern consisting of absorption parts and reflective parts. Optical images reflected by the reflective mask produced in such manner are transcribed onto a semiconductor substrate via a catoptric system.
Description of Reflectance and Film Thickness of Absorption Layer of EUV Mask
With an exposure method using the catoptric system, a shadow of a pattern itself is generated when the film thickness of the absorption layer is large, since light is emitted at an incidence angle (ordinarily 6°) inclined by a predetermined angle from a perpendicular direction with respect to the mask surface. Since the reflection intensity at the shadowed part is smaller than a part that is not shadowed, a reduced contrast is obtained, resulting in a transcription pattern having blurred edge parts and deviation from a designed size. This is referred to as shadowing, and is a fundamental problem for reflective masks.
In order to prevent blurred edge parts of the pattern and deviation from the designed size, reducing the film thickness of the absorption layer and lowering the height of the pattern are effective. However, reducing the film thickness of the absorption layer results in deterioration in the light-shielding ability at the absorption layer, reduction in transcription contrast, and deterioration in accuracy of the transcription pattern. If the absorption layer is too thin, the contrast necessary for maintaining accuracy of the transcription pattern cannot be obtained. Thus, since it becomes a problem if the film thickness of the absorption layer is too large or too small, the film thickness at present is generally set between 50 to 90 nm, and the reflectance of EUV light at the absorption layer is about 0.5 to 2%.
Description of Multiple Exposures of Adjacent Chips
On the other hand, when forming a transcription circuit pattern on a semiconductor substrate using a reflective mask, chips with multiple circuit patterns are formed on a single layer of semiconductor substrate. In some cases, there is an overlapping area of chip outer peripheral portions of adjacent chips. This is due to chips being arranged in high density for the purpose of improving productivity based on an intention of increasing the number of chips that can be obtained from a single sheet of wafer. In this case, such area is exposed for multiple times (at maximum four times) (multiple exposures). The chip outer peripheral portions of transcription pattern correspond to outer circumferential portions of the mask, and are portions where an absorption layer is formed, ordinarily. However, as described above, since the reflectance of EUV light at the absorption layer is about 0.5 to 2%, there has been a problem of the chip outer peripheral portions being sensitized due to the multiple exposures. Therefore, it has become necessary to provide, at the chip outer peripheral portion on the mask, an area (hereinafter, referred to as a light-shielding frame) that has higher light-shielding ability against EUV light than an ordinary absorption layer.
In order to solve such a problem, reflective masks having a light-shielding frame with high light-shielding ability against exposure light source wavelengths are proposed, including a reflective mask having formed therein a trench that is dug from an absorption layer into a multilayered reflective layer of the reflective mask, a reflective mask having formed therein a film that has a larger film thickness than an absorption layer at a circuit pattern area, and a reflective mask in which reflectance of a multilayered reflective layer is reduced by laser irradiation or ion implantation on the reflective mask (cf. Patent Literature 1 and 2).